This invention relates to diagnostic x-ray apparatus and, particularly, to a system that is capable of performing hybrid digital subtraction angiography procedures.
Hybrid digital subtraction angiography is described in detail in U.S. patent application Ser. No. 371,683, filed Apr. 26, 1982, now U.S. Pat. No. 4,482,918 . This patent is assigned to the assignee of the present application. The object of digital subtraction angiography is to produce a visible image of a blood vessel whose lumen is occupied by an x-ray opaque medium in which image soft tissue and boney structures which might otherwise obscure the vessel are cancelled out. In hybrid digital subtraction angiography x-ray images of the anatomy of interest are made by exposing the patient to x-ray beams having different average energy levels, that is, having two different narrow-x-ray spectral bands. The so-called low energy exposures are made with comparatively low peak kilovoltage (kVp), such as 60 to 90 kVp, applied to the x-ray tube anode. The so-called high energy exposures are made with, typically, 130 to 140 kVp applied to the x-ray tube anode. The x-ray tube current or milliamperage (MA) is higher for the low energy exposures than for the high energy exposures. The duration of the low energy exposures may be longer or shorter than the duration of the high energy exposures, depending on the density of the anatomical region being examined, but usually the low energy exposures have the longer duration. In the hybrid subtraction mode used to illustrate the invention herein, the patient is arranged between an x-ray tube and an x-ray image intensifier whose optical output image is viewed by a television (TV) camera. The x-ray tube power supply is adapted to switch the kVp applied to the x-ray tube anode between low and high levels very rapidly. During low energy exposures an x-ray filter is inserted in the beam to filter out or attenuate radiation having energy below the low energy spectral band and during the high energy exposures a different filter is inserted in the beam to filter out or attenuate radiation having energy below that of the high energy spectral band. In the exemplary hybrid subtraction mode, a low energy mask image is obtained prior to the time that the x-ray contrast medium which has been injected somewhere in the blood vessel of the patient reaches the blood vessel of interest. The digitized picture element (pixel) data representative of the low energy mask image are stored on magnetic disk. As soon as the low energy mask image is acquired the high energy mask image is made and its pixel data are stored. The mask images are made during what is called the precontrast time. It is desirable that the two mask images be made as close together as possible so that there will be no adverse effect produced by voluntary or involuntary movement of the patient's anatomy between the x-ray exposures. After the mask images are obtained, closely successive low and high energy exposure pairs are made through the pre-contrast time and through the post-contrast time during which the contrast medium is flowing through the blood vessel of interest. The raw digital pixel data representative of these images are stored on magnetic disk. In a subsequent reprocessing procedure, the data are accessed and the low and high energy mask images are subtracted from the subsequent low and high energy images, respectively, and the resulting sequence of low and high energy difference images data are stored. Subtraction causes anything that remains constant throughout the sequence of images to be cancelled and lets data representative of the contrast medium and anything that changes remain. The low energy difference images data and the high energy difference images data are then summed to produce two sets of data one of which represents the sum of the low energy images and the other of which represents the sum of the high energy images. The low energy image data set is then multiplied by a weighting factor and the high energy image data set is multiplied by another weighting factor. These factors are chosen so that when the sets of multiplied data are subtracted, data representative of motion of a specific material are substantially cancelled. After weighting the two data sets, one is subtracted from the other and the resulting set of data represents the image of the contrast medium in the blood vessel.
The apparatus described herein can be used to perform procedures other than hybrid digital subtraction angiography. For example it can perform ordinary temporal subtraction and energy subtraction procedures which require no further description for those skilled in the digital fluorography art.
Several problems that are connected with performing hybrid subtraction angiography have not been solved satisfactorily heretofore. The first problem is to maximize spectral-energy separation. The second problem is to minimize the total x-ray exposure time to prevent patient motion from interfering with the cancellation process. A third problem is to prevent damage to the x-ray tube which will occur if the energy input to the tube is too great during an exposure sequence.
There are two thermal factors that must be considered in rotating anode x-ray tubes. Typically, the temperature of the bulk of the x-ray tube target or rotating anode should not be allowed to exceed about 1100.degree. C. or else the target may warp or conduct so much heat to the anode bearing that they will be damaged. Another factor to be considered is that when the electron beam current exceeds a certain value while the high kVp is applied to the anode of the x-ray tube there may be melting of the target where the beam is focused on it which means that there must be assurance that the temperature at the focal spot will not exceed about 3000.degree. C. for rhenium alloy coated tungsten targets which are most commonly used in high capacity rotary anode x-ray tubes at the present time.
In prior art digital subtraction angiography systems a single x-ray exposure was made, usually a low energy exposure, that is, an exposure using low kVp on the x-ray tube anode and relatively high x-ray tube MA. An automatic exposure control (AEC) was used to terminate the exposure when the desired x-ray dosage was accumulated. Means were provided for measuring the automatically terminated exposure time interval and this time was stored and used to govern the length of all subsequent high and low energy exposure intervals. One of the problems with using the same exposure time for the low and high energy exposures is that sometimes the optical version of the x-ray image is too bright for the TV camera and at other times it is not bright enough. The former way around this problem was to have the user make several trial exposures and adjust the exposure time until the proper light level to the TV camera was obtained. Unfortunately, while exposure time is being optimized the thermal load on the x-ray tube target may be increased, resulting in damage to the target.
Minimizing the time between high and low energy exposures is important. In conventional practice, each low energy x-ray exposure and each high energy x-ray exposure is initiated in synchronism with the TV camera vertical blanking pulses and the camera target is not read out until the first blanking pulse occurs following the TV frame in which the exposure ends. There is no target readout during the x-ray exposure. A low energy exposure, for example, would start with a vertical blanking pulse and might end within a single TV frame time or it might extend over several frame times and terminate somewhere within a frame time. Readout of the TV camera pickup tube is blanked during the x-ray exposure so the image is fully formed before the TV tube beam is allowed to scan the camera tube target. When the exposure ends within a particular TV frame there is a delay until the next vertical blanking pulse occurs to initiate the next frame time during which the TV pickup tube target is read out to produce the analog video signals representative of the image. The ensuing high energy exposure is started concurrently with the first vertical blanking pulse that was coincident with the end of the TV target readout frame. The delay between the end of the low energy exposure and the next ensuing blanking pulse that started readout did not represent the minimum time that could be obtained between the end of the low energy exposure and the beginning of the high energy exposure.